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1.
Geografisk Tidsskrift—Danish Journal of Geography 110(2):337–355, 2010

In northern Greenland, the Cape Grinnell beach ridge plain offers a 9,000year multi-proxy record for isostatic recovery, storm history, and the hydrological changes related to precipitation and slope evolution. The chronology of uplifted beach ridges is constrained by ten geological 14C ages on shell and sea mammal bones and eleven upper limiting ages from archaeological sites that span the last 3,000 years. Beach ridges formed under the influence of open water storms with renewed frequency and intensity ca. 3 ka and 1 ka ago. A lack of shell may reflect cooler sea surface temperatures. The presence and absence of ice can be inferred by push-features. Three intervals of heightened precipitation produced extensive fan deltas: (a) after 9 ka BP (b) prior to 4.5 ka BP and (c) during the Little Ice Age (AD 1350–1900). Active solifluction lobes and colluvia cover beach ridge deposits that are between 9 and 7 ka old.  相似文献   

2.
The last British Ice Sheet: growth, maximum extent and deglaciation   总被引：2，自引：0，他引：2  
The growth, maximum lateral extent and deglaciation of the last British Ice Sheet (BIS) has been reconstructed using sediment, faunal and stable isotope methods from a sedimentary record recovered from the Barra Fan, north-west Scotland. During a phase of ice sheet expansion postdating the early "warmth" of Marine Isotope Stage 3 (MIS 3), ice rafting events, operating with a cyclicity of approximately 1500 years, are interspersed between warm, carbonate-rich interstadials operating with a strong Dansgaard-Oeschger (D-O) cyclicity. The data suggest that the BIS expanded westwards to the outer continental shelf break shortly after 30 Ky BP (before present) and remained there until about 15 Ky BP. Within MIS 2, as the ice sheet grew to its maximum extent, the pronounced periodicities which characterize MIS 3 are lost from the record. The exact timing of the Last Glacial Maximum is difficult to define in this record; but maxima in Neogloboquadrina pachyderma (sinistral) Ø18O are observed between 21-17 Ky BP. A massive discharge of ice-rafted detritus, coincident with Heinrich event 1, is observed at about 16 Ky BP. Deglaciation of the margin is complete by about 15 Ky BP and surface waters warm rapidly after this date.  相似文献   

3.
Earlier work in northeast Greenland has suggested a limited advance of the Greenland Ice Sheet during the Last Glacial Maximum (LGM). However, this concept has recently been challenged by marine geological studies, indicating grounded ice on the continental shelf at this time. New 10Be‐ages from the Store Koldewey island, northeast Greenland, suggest that unscoured mountain plateaus at the outer coast were covered at least partly by cold‐based ice during the LGM. It is, however, still inconclusive whether this ice was dynamically connected to the Greenland Ice Sheet or not. Regardless of the LGM ice sheet extent, the 10Be results from Store Koldewey add to a growing body of evidence suggesting considerable antiquity of crystalline unscoured terrain near present and Pleistocene ice sheet margins.  相似文献   

4.
Detailed mapping and elevation measurements of glacial lake shorelines in the Flin Flon region has permitted the reconstruction of 6 well-defined levels of Lake Agassiz formed around 8.3 ka to 7.9 ka 14C BP. The Stonewall, The Pas, Gimli, Grand Rapids, Drunken Point and Ponton paleo-water planes have been tilted upward to the northeast in the Holocene, with gradients decreasing, from the highest to the lowest level, from about 0.34 m km-1 to 0.22 m km-1 in the study area. The Setting level, lower than the Ponton but less well defined, is also documented here for the first time. This mapping conclusively refutes the view, entrenched in the literature from the 1890's to the 1960's, that there has been negligible differential uplift in the region following final drainage of Lake Agassiz. The finding has major consequences regarding correlation of glacial lakes across the mid-continent, the post-glacial history of large lakes in the region, and for interpretation of earth rheology and its implications for ice sheet reconstruction.  相似文献   

5.
Sedimentological, geochemical and magnetic data in a ~ 7-m sequence from Marboré Lake (2612 m asl, central Pyrenees) provide information about environmental variability since the last glacier retreat (14.6 cal ka BP) in high-altitude Pyrenean environments. The sediment sequence is composed of millimeter- to centimeter-thick rhythmites made of finer greyish laminae and coarser-grain, carbonate-bearing laminae arranged in varied patterns throughout the sequence. Finer laminae are interpreted as deposition during periods of predominantly ice-covered conditions, whereas coarser, carbonate-bearing sediments reflect periods of higher runoff. The age model, based on 13 14C dates and a reservoir effect assessed with 210Pb and 137Cs, is coherent with known synchronous vegetation changes across the Pyrenees. Warmer intervals such as GI-1 (14.6–12.8 cal ka BP, Bølling/Allerød period), 10.4–8.2 cal ka BP in the Early Holocene, 7.5–5.2 cal ka BP in the Mid Holocene and the Medieval Climate Anomaly (AD 900–1300), are characterized by peaks in productivity and higher carbonate preservation. Deposition during colder periods such as GS-1 (12.8–11.7 cal ka BP), the Neoglacial (ca. 5.2–3.5 cal ka BP) and the Little Ice Age (last 400 years) show an increase in finer laminae. The presence of magnetite throughout the whole section suggests that Marboré Lake maintained predominantly oxic conditions since its formation. Changes in magnetic properties and the increase in magnetite from 3.5 cal ka BP to present, however, indicate a more oxic environment at the lake bottom during the last few millennia. The occurrence of Pb concentration peaks in sediments of Roman and modern age demonstrates the global distribution of heavy metal deposition, even into high-mountain lakes.  相似文献   

6.
李栓科 《极地研究》1995,7(4):10-19
拉斯曼丘陵在渐新世已被冰盖覆没，晚渐新世冰盖厚度最大，自中新世开始，冰盖逐渐减薄后退，但该丘陵仍为冰盖占据。更新世冰川作用规模不及第三纪。１８ｋａＢ．Ｐ．该丘陵区覆冰厚度超过１７０ｍ，冰盖前缘仅增厚３０ｍ左右。冰盖后退出露基岩约在１０．０ｋａＢ．Ｐ．前后，自９．４１～６．５ｋａＢ．Ｐ．，冰川以２～３ｍ／ａ的速率后退，岛屿区全部出露；自６．５～５．０ｋａＢ．Ｐ．，冰川后退速率减为１．０～１．５ｍ／ａ，丘陵区裸露５０％以上；约自５．０ｋａＢ．Ｐ．开始，丘陵区几乎全部出露，冰川冰盖范围与今基本相同。  相似文献   

7.
We used ostracod species assemblages and their δ18O values in a 32-m sediment core from Lake Qinghai, China, along with information from cores collected at other sites in the lake, to infer lake evolution and hydroclimate changes since the last glacial. Dominant ostracod species Ilyocypris bradyi and its low δ18O values showed that Lake Qinghai was small in size or even consisted of several playa lakes, and the 1F core site could have even been in a wetland setting, under cold and dry climate conditions before 15.0 ka. Presence of Limnocythere inopinata with low δ18O values, and absence of I. bradyi after 15.0 ka, indicate the lake area increased or that the playas merged. The decrease or disappearance of ostracods with high δ18O values showed that the lake shrunk under dry climate from 12.0 to 11.6 ka. After 11.6 ka, hydroclimate shifts inferred from ostracod species changes (Eucypris mareotica and L. inopinata) and their δ18O values were as follows: (1) 11.6–7.4 ka—larger, but still small lake area with greater moisture availability under primarily dry climate conditions, (2) 7.4 to 3.2 ka—increasing lake level under a warmer and wetter climate, and (3) 3.2 ka to present—stable, large, brackish lake. The low ratio of lake water volume to runoff, and close proximity of the core site to freshwater input from the river mouth would have resulted in relatively lower ostracod δ18O values when Lake Qinghai was small in area during the interval from 32.0 to 15.0 ka. Lower ostracod δ18O values during interstadials and throughout the entire Last Glacial Maximum and early deglacial (ca. 24.0–16.0 ka) were caused by a greater contribution of seasonal meltwater from ice or snow and low incoming precipitation δ18O values related to cold climate conditions in the region at that time.  相似文献   

8.
The Paleoclimates from Arctic Lakes and Estuaries (PALE) project has chosen to conduct high resolution data-model comparisons for the Arctic region at 21 and 10 (calendar) ka BP. The model simulations for 21, 10, and 0 ka BP were conducted with the GENESIS 2.0 GCM. The 10 ka BP simulation was coupled to the EVE vegetation model. The primary boundary conditions differing from present at 21 ka BP were the northern hemisphere ice sheets and lower CO2, and at 10 ka BP were the orbital insolation and smaller northern hemisphere ice sheets. The purpose of this article is to discuss the hydrological consequences of these simulations.At the Last Glacial Maximum (21 ka BP) the large ice sheets over North America and Eurasia and the lower CO2 levels produced a colder climate than present, with less precipitation throughout the Arctic, except where circulation was altered by the ice sheets. At 10 ka BP greater summer insolation resulted in a warmer and wetter Beringia, but conditions remained cold and dry in the north Atlantic sector, in the vicinity of the remnant ice sheets. Less winter insolation at 10 ka BP resulted in colder and drier conditions throughout the Arctic. Precipitation - evaporation generally correlated with precipitation except where changes in the surface type (ice sheets, vegetation at 10 ka BP, or sea level at 21 ka BP) caused large changes in the evaporation rate. The primary hydrological differences (from present) at 21 and 10 ka BP correlated with the temperature differences, which were a direct result of the large-scale boundary condition changes.  相似文献   

9.
Detailed geomorphological mapping provides evidence for at least three phases of glaciation in the Parque Natural Lago de Sanabria, in northwest Spain. The most extensive glaciation was characterised by a large plateau ice cap. A combination of geomorphological evidence and glacier modelling indicates that this ice cap covered an area of more than 440 km2, with a maximum ice thickness of c. 300 m and outlet glaciers reaching as low as 1000 m. This represents the largest ice mass in Iberia outside the Pyrenees and one of the largest in the mountains of southern Europe and the Mediterranean region. Radiocarbon dates from the base of lacustrine sequences appear to suggest that the most extensive phase of ice-cap glaciation occurred during the last cold stage (Weichselian) with deglaciation occurring before 14–15 ka 14C BP. A second phase of glaciation is recorded by the moraines of valley glaciers, which may have drained small plateau ice caps; whilst a final phase of glaciation is recorded by moraines in the highest cirques.  相似文献   

10.
Isotopic records of aquatic cellulose are becoming increasingly important for palaeohydrological reconstructions, but widespread application of this climate proxy is hampered by minerogenic contamination that affects oxygen isotope measures in cellulose. Few records of isotopes in aquatic cellulose are available from palaeoclimate archives in the Southern Hemisphere. In this study, we used a new bulk cellulose extraction method and determined the oxygen (δ18O) and carbon (δ13C) isotope values in cellulose from a Holocene lake sediment core segment (7.2–1.1 cal ka BP) from Lake Pupuke, Auckland, New Zealand. Isotope values from modern, potential sources of sedimentary cellulose revealed the aquatic origin of the cellulose extracted from the core, and hence enabled inference of past lake water δ18O values from the δ18O of measured cellulose in the core. A shift to a more positive water balance in the lake was identified around 2.8 cal ka BP by a decrease in inferred lake water δ18O values. At that time, greater epilimnetic primary productivity is indicated by the higher δ13C values of sedimentary cellulose. Greater divergence between the δ13C values of cellulose and bulk organic matter suggests stronger stratification of the lake, likely caused by greater freshwater input. We discuss a possible link to a solar minimum that occurred at that time.  相似文献   

11.
The occurrence of tors within glaciated regions has been widely cited as evidence for the preservation of relic pre-Quaternary landscapes beneath protective covers of non-erosive dry-based ice. Here, we test for the preservation of pre-Quaternary landscapes with cosmogenic surface exposure dating of tors. Numerous granite tors are present on summit plateaus in the Cairngorm Mountains of Scotland where they were covered by local ice caps many times during the Pleistocene. Cosmogenic 10Be and 26Al data together with geomorphic relationships reveal that these landforms are more dynamic and younger than previously suspected. Many Cairngorm tors have been bulldozed and toppled along horizontal joints by ice motion, leaving event surfaces on tor remnants and erratics that can be dated with cosmogenic nuclides. As the surfaces have been subject to episodic burial by ice, an exposure model based upon ice and marine sediment core proxies for local glacial cover is necessary to interpret the cosmogenic nuclide data. Exposure ages and weathering characteristics of tors are closely correlated. Glacially modified tors and boulder erratics with slightly weathered surfaces have 10Be exposure ages of about 15 to 43 ka. Nuclide inheritance is present in many of these surfaces. Correction for inheritance indicates that the eastern Cairngorms were deglaciated at 15.6 ± 0.9 ka. Glacially modified tors with moderate to advanced weathering features have 10Be exposure ages of 19 to 92 ka. These surfaces were only slightly modified during the last glacial cycle and gained much of their exposure during the interstadial of marine Oxygen Isotope Stage 5 or earlier. Tors lacking evidence of glacial modification and exhibiting advanced weathering have 10Be exposure ages between 52 and 297 ka. Nuclide concentrations in these surfaces are probably controlled by bedrock erosion rates instead of discrete glacial events. Maximum erosion rates estimated from 10Be range from 2.8 to 12.0 mm/ka, with an error weighted mean of 4.1 ± 0.2 mm/ka. Three of these surfaces yield model exposure-plus-burial ages of 295− 71+ 84, 520− 141+ 178, and 626− 85+ 102 ka. A vertical cosmogenic nuclide profile across the oldest sampled tor indicates a long-term emergence rate of 31 ± 2 mm/ka. These findings show that dry-based ice caps are capable of substantially eroding tors by entraining blocks previously detached by weathering processes. Bedrock surfaces and erratic boulders in such settings are likely to have nuclide inheritance and may yield erroneous (too old) exposure ages. While many Cairngorm tors have survived multiple glacial cycles, rates of regolith stripping and bedrock erosion are too high to permit the widespread preservation of pre-Quaternary rock surfaces.  相似文献   

12.
A coarse-resolution, absolutely dated isotope record spanning the last 180 ka was constructed from aragonite-rich fecal pellets in a sediment core from Lake Urmia, Iran. The isotope record is not continuous as a consequence of detrital calcite that contaminates the pellets at certain depths. An isotopic correction was applied to samples > 50% aragonite, using the δ18O values of calcite-only pellets. Absolute dates were derived from U-Th analyses on pellets > 80% aragonite. The resultant δ18O record and sediment lithofacies confirm that the Penultimate Glacial was cold and dry, with pronounced interstadials at ca. 150 and >175 ka. Deglaciation began at ca. 130 ka, but climate was unstable and supported a probable no-analog vegetation assemblage. The onset of true interglacial conditions occurred at ca. 126 ka. Peak moisture conditions lasted for 5–6 ka before the climate became drier, with the loss of some tree taxa towards the end of the Last Interglacial. A period of greater moisture occurred between 116 and 108 ka, overlapping in timing with interstadial conditions recorded in the Soreq Cave speleothem record. In general, stadial to interstadial transitions (OIS 5d-5c, 5b-5a) are marked by decreasing δ18O values and rising lake levels. The nature of the OIS 5 to OIS 4 transition cannot be resolved in this study because of the loss of 5 m of core and imprecise dates. According to the present chronology, δ18O values, and inferred lake levels, arid conditions began at 60 ka and persisted until Termination I. Episodic increases in moisture, characterized by layers of aragonite-rich pellets, ostracodes, and the pollen of trees and/or aquatic plants, occurred during the Last Glacial. Deglaciation associated with the Late-glacial Interstadial began at 14 ka but was interrupted by a marked Younger Dryas event that resulted in renewed aridity. The Holocene began at approximately 10 ka. Despite large errors in the chronology, the overall timing and pattern of effective moisture matches the record from Soreq Cave, Israel, suggesting that the two regions were climatically in-phase.  相似文献   

13.
Uncertainty remains if ice–free marginal areas existed on the west coast of Svalbard during the Late Weichselian. Field mapping and correlation to well dated raised beach sequences on nearby Brøggerhalvøya reveal the existence of two generations of raised beach deposits on northern Prins Karls Forland. Distinct beach ridges rise up to the inferred Late Weichselian marine limit at 18 m a.s.l. Discontinuous pre–Late Weichselian beach deposits rise from the Late Weichselian marine limit up to approximately 60 m a.s.l. Expansion of local glaciers during the Late Weichselian is indicated by the limited distribution of a till that overlies parts of the older beach sequence. Stratigraphic data and chronological control indicate deposition in a shallow marine environment before 50 ka bp . Correlation to stratigraphic sites on western Svalbard suggests deposition at c . 70 ±10 ka. Glaciotectonic structures disclose expansion of local glaciers into the For–landsundet basin during stage 4 or late stage 5 high relative sea level. Palaeotemperature estimates derived from amino acid ratios indicate that during the time interval c . 70 to 10 ka the area was exposed to cold subaerial temperatures with low rates of racemization. Pedogenesis and frost–shattered clasts at the contact between c . 70 ka deposits and Holocene deposits further indicate a prolonged period of subaerial polar desert conditions during this time interval. The evidence suggests that the Barents Sea ice sheet did not extend across northern Prins Karls Forland during the Weichselian. It is inferred that during the Late Weichselian, ice was drained throughout the major fjords on the west coast of Svalbard and that relatively large marginal areas experienced polar desert conditions and minor expansions of local glaciers.  相似文献   

14.
Two cores were recovered from raised peat bogs on the tropical northern Leizhou Peninsula, south China. Multiple sediment variables including organic matter (OM) content, the stable carbon isotope signature of OM, low-frequency magnetic susceptibility and degree of humification, indicate that the regional paleoclimate played an important role in determining the nature of peat that accumulated. Based on comparison with other climate proxies, the bulk peat δ13C record was interpreted as an indicator of variation in the East Asian (EA) summer monsoon, and to a lesser extent, the Indian summer monsoon, during the last glacial period between ~49 and 10 cal ka BP. More negative bulk δ13C values reflect wetter and warmer conditions, and thus a strong EA summer monsoon. More positive values indicate drier and cooler conditions. A warm and wet period occurred between ~46 and 28 cal ka BP, implying a strengthening of the EA summer monsoon. A climate shift occurred at ~22 cal ka BP and the driest and coldest period appeared between ~19 and 16 cal ka BP, suggesting weakening of the EA summer monsoon. After ~12 cal ka BP, climate shifted towards wetter and warmer conditions again. It has been suggested that variations in orbitally induced solar insolation played a role in the last glacial climate of the study region. Several millennial—scale arid and cold phases characterized by C4 plants, or by more positive δ13C values during periods when C3 plants dominated, show agreement with the Greenland GISP2 ice core and the Chinese stalagmite records. Interactions between high northern latitude cold air advection and summer moisture transported across the tropical ocean, and the migration of the mean position of the Intertropical Convergence Zone (ITCZ) would have favored these millennial–scale phases. Additionally, changes in heat transport to the North Atlantic would also have influenced climate in the region.  相似文献   

15.
At high‐latitude continental margins, large‐scale submarine sliding has been an important process for deep‐sea sediment transfer during glacial and interglacial periods. Little is, however, known about the importance of this process prior to the arrival of the ice sheet on the continental shelf. Based on new two‐dimensional seismic data from the NW Barents Sea continental margin, this study documents the presence of thick and regionally extensive submarine slides formed between 2.7 and 2.1 Ma, before shelf‐edge glaciation. The largest submarine slide, located in the northern part of the Storfjorden Trough Mouth Fan (TMF), left a scar and is characterized by an at least 870‐m‐thick interval of chaotic to reflection‐free seismic facies interpreted as debrites. The full extent of this slide debrite 1 is yet unknown but it has a mapped areal distribution of at least 10.7 × 103 km2 and it involved >4.1 × 103 km3 of sediments. It remobilized a larger sediment volume than one of the largest exposed submarine slides in the world – the Storegga Slide in the Norwegian Sea. In the southern part of the Storfjorden TMF and along the Kveithola TMF, the seismic data reveal at least four large‐scale slide debrites, characterized by seismic facies similar to the slide debrite 1. Each of them is ca. 295‐m thick, covers an area of at least 7.04 × 103 km2 and involved 1.1 × 103 km3 of sediments. These five submarine slide debrites represent approximately one quarter of the total volume of sediments deposited during the time 2.7–1.5 Ma along the NW Barents Sea. The preconditioning factors for submarine sliding in this area probably included deposition at high sedimentation rate, some of which may have occurred in periods of low eustatic sea‐level. Intervals of weak contouritic sediments might also have contributed to the instability of part of the slope succession as these deposits are known from other parts of the Norwegian margin and elsewhere to have the potential to act as weak layers. Triggering was probably caused by seismicity associated with the nearby and active Knipovich spreading ridge and/or the old tectonic lineaments within the Spitsbergen Shear Zone. This seismicity is inferred to be the main influence of the large‐scale sliding in this area as this and previous studies have documented that sliding have occurred independently of climatic variations, i.e. both before and during the period of ice sheets repeatedly covering the continental shelf.  相似文献   

16.
High-resolution quantitative analysis of ostracod assemblages from 4.3-m-thick freshwater tufa-rich sediments, deposited during the last 12.8 ka in Lake Sinijärv, northern Estonia, yielded information on water level, trophic state conditions, and temperature changes since the late glacial. AMS 14C dates from aquatic mosses provided time constraints on the palaeoenvironmental development of the region. In the ostracod assemblage structure, four faunal zones (OFZ) were determined. The most significant change in the ostracod fauna occurred at 10,590 cal. y BP, when a typical littoral, polythermophilic fauna was replaced by a mostly sublittoral, species-rich meso- to stenothermophilic fauna. The ostracod data indicate two major low-water-level periods in the lake at 12,800–10,590 and 7,600–3,700 cal. y BP. Sediment analysis indicates the most intensive tufa precipitation occurred during these low stand periods, rather than during the warmest climate in Estonia between 8,000 and 4,500 cal. y BP. The late glacial low water level in the groundwater-fed Lake Sinijärv at 12,800–10,590 cal. y BP coincides partly with the regression in the Lake Peipsi basin (14,000–12,100 cal. y BP) and with the last drainage event of the Baltic Ice Lake at 11,600 cal. y BP. The low-water-level period in Lake Sinijärv occurred earlier than in lakes in the SE sector of Scandinavian glaciation. The change from low to high water level in Lake Sinijärv at 10,590 cal. y BP preceded the first post-glacial transgression events in the small lowland lakes of Estonia, southern Sweden, Poles`ye in Belarus, and Valday in NW Russia. In general, the mid-Holocene low-water-level period in Lake Sinijärv between 7,600 and 3,700 cal. y BP is concurrent with the regressions in the lakes of the SE sector of Scandinavian glaciation.  相似文献   

17.
Climate change in the northeastern United States has been inferred for the last deglaciation to middle Holocene (∼16,600 to 6000 calendar years ago) using multi-proxy data (total organic matter, total carbonate content, δ18 O calcite and δ13 C calcite) from a 5 m long sediment core from Seneca Lake, New York. Much of the regional postglacial warming occurred during the well-known Bolling and Allerod warm periods (∼14.5 to 13.0 ka), but climate amelioration in the northeastern United States preceded that in Greenland by ∼2000 years. An Oldest Dryas climate event (∼15.1 to 14.7 ka) is recognized in Seneca Lake as is a brief Older Dryas (∼14.1 ka) cold event. This latter cold event correlates with the regional expansion of glacial Lake Iroquois and global meltwater pulse IA. An increase in winter precipitation and a shorter growing season likely characterized the northeastern United States at this time. The Intra-Allerod Cold Period (∼13.2 ka) is also evident supporting an “Amphi-Atlantic Oscillation” at this time. The well-known Younger Dryas cold interval occurred in the northeastern United States between 12.9 and 11.6 ka, consistent with ice core data from Greenland. In the Seneca Lake record, however, the Younger Dryas appears as an asymmetric event characterized by an abrupt, high-amplitude beginning followed by a more gradual recovery. Compared to European records, the Younger Dryas in the northeastern United States was a relatively low-amplitude event. The largest amplitude and longest duration anomaly in the Seneca Lake record occurs after the Younger Dryas, between ∼11.6 and 10.3 ka. This “post-Younger Dryas climate interval” represents the last deglacial climate event prior to the start of the Holocene in the northeastern United States, but has not been recognized in Greenland or Europe. The early to middle Holocene in the northeastern United States was characterized by low-amplitude climate variability. A general warming trend during the Holocene Hypsithermal peaked at ∼9 ka coincident with maximum summer insolation controlled by orbital parameters. Millennial- to century-scale variability is also evident in the Holocene Seneca Lake record, including the well-known 8.2 ka cold event (as well as events at ∼7.1 and 6.6 ka). Hemispherical cooling during the Holocene Neoglacial in the northeastern United States began ∼5.5 ka in response to decreasing summer insolation.  相似文献   

18.
The overall pattern of deglaciation of the southern part of the Scandinavian Ice Sheet has been considered established, although details of the chronology and ice sheet dynamics are less well known. Even less is known for the south Swedish Upland because the area was deglaciated mostly by stagnation. Within this area lies the conspicuous Vimmerby moraine, for which we have used the terrestrial cosmogenic nuclide (10Be) exposure dating technique to derive the exposure age of six glacially transported boulders. The six 10Be cosmogenic ages are internally consistent, ranging from 14.9 ± 1.5 to 12.4 ± 1.3 ka with a mean of 13.6 ±0.9 ka. Adjusting for the effects of surface erosion, snow burial and glacio-isostatic rebound causes the mean age to increase only by c. 6% to c. 14.4± 0.9 ka. The 10Be derived age for the Vimmerby moraine is in agreement with previous estimates forthe timing of deglaciation based on radiocarbon dating and varve chronology. This result shows promise for further terrestrial cosmogenic nuclide exposure studies in southern Sweden.  相似文献   

19.
Since 1980, speleothems, mollusc shells and vertebrate teeth have been dated at the Uranium Series Radiometric Laboratory at Oslo University: To date, 44 samples have been dated with 230Th/232Th ratios ≥20. As found with other speleothem dates, they tend to cluster into discreet groups, roughly corresponding to ice-free periods of the Late Pleistocene. The direct connection between speleothem depositon and ice-free surface conditions is not always exactly correct, and it is preferable to operate with ‘speleothem chronozones’ when discussing these phenomena. Speleothem chronozones have been found in Norway for 0 to 12 ka BP (Holocenc), 90 to 130 ka BP (Rana-Fauske Chronozone), 170 to 200 ka BP (Gildeskål Chronozone), and greater than 350 ka BP. The latter samples (>350 ka BP) were all younger than 1.5 ma BP, and they most likely fall within the range of 0.3 to 0.8 ma BP, based on regional means of initial 234Usol;238U. Some samples which were contaminated with detrital Thorium gave ‘corrected’ ages (assuming initial 230Th/232Th = 1.5) of 36, 40, 50 and 60 ka BP, respectively. These dates may correlate with the Weichselian Interstadial complex (Ålesund/Sandnes, 30 to 39 ka BP; Jæren/Nygaard, 40 to 50 ka BP; and Brørup/Jämtland, 58 to 68 ka BP) chronozones. More work is required to clarify the problems and increase the statistical significance of the findings.  相似文献   

20.
The Bunger Hills in East Antarctica occupy a land area of approximately 400 km2. They have been exposed by Holocene retreat of the Antarctic ice sheet and its outlet glaciers. The accompanying sea level rise flooded the marine inlets that now separate the northern islands and peninsulas from the major part of the hills. During deglaciation the continental ice sheet margin retreated south‐eastwards with several temporary halts, during which ice‐dammed lakes were formed in some valleys. These lakes were maintained long enough to permit formation of beaches of sand and gravel, and for the erosion of shore platforms and low cliffs in bedrock. Around the western end of Fish Tail Bay impressive shoreline features 20 m above sea level define a former ice‐dammed lake that was 5.5 km long. A similar 7 km long former ice‐dammed lake was formed at Lake Dolgoe. The more extensive and deeper glacial lake is revealed by well‐developed and preserved shoreline features cut at 29 m which is 16 m above present lake level. In addition, several small ice‐dammed lakes existed temporarily near Lake Shchel and in the valley to the west. Lake Fish Tail existed more than 6,900 14C years ago and Lake Shchel probably more than 6,680 14C years ago. It is inferred that the shore platforms and beaches were formed by lake ice and wave action over considerable periods when the lakes were impounded by steep cold ice margins. There appears to have been a balance between meltwater input and evaporative loss from the lakes in the cold dry continental climate. There is no evidence for rapid lake level fluctuations, and there was very little input of clastic sediment. This resulted in poor development of deltaic and rhythmically laminated lake floor deposits. It is suggested that such deposits are more characteristic of ice‐dammed lakes formed in association with wet‐based temperate ice than those associated with dry‐based polar ice.  相似文献   

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